Molding sand appropriate for the fabrication of cores and molds

ABSTRACT

The molding sand comprises hollow microspheres of aluminum silicate, preferably with an aluminum content between 15 and 45% by weight, a wall thickness between 3 and 10% of the particle diameter and a particle size between 10 and 350 μm. These sands are useful to manufacture low density cores with good “veining” and penetration characteristics, moreover maintaining the mechanical properties of the core obtained. These cores are useful in the manufacture of iron casting.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a 371 of PCT/ES97/00314 filed on Dec. 26, 1997.

FIELD OF THE INVENTION

This invention is related to the manufacture of iron casting and,specifically, it refers to a molding sand for casting, suitable formanufacturing cores and chill molds, incorporating hollow microspheresof aluminum silicate.

BACKGROUND OF THE INVENTION

The iron casting obtained by using cores manufactured with molding sand,generally have a series of defects in their shape, such that it isnecessary to subject them to machining to obtain a dimensionally correctpiece. These defects are produced due to the heating the core suffersdue to the effect of the molten metal poured over it, provoking itsexpansion and hence, the appearance of fissures on its surface. Themolten metal penetrates these fissures, hence forming a kind ofpartition wall or laminas on the surface of the piece obtained. Thisundesired effect is known “veining” or “rat's tail”.

At present, the cores are manufactured using molding sands and gas- orheat-cured resins, or self-curing resins, together with additivesdestined to improve the characteristics of the piece obtained.

To prevent the formation of “veining”, a series of techniques are knownand used, such as:

The Use of Iron Oxide as an Additive

The iron oxides used as additives, are destined to minimize the problemscreated by the expansion of the silica contained in the sands, beingused for such a purpose red, black, yellow iron oxides or iron oxidefrom Sierra Leone, which are incorporated to the mixture in percentagesvarying from 1 to 3%. These oxides act as a factor for the formation offeyalite, such that the “veining” is minimized during the formation ofthe fissure. Nevertheless, this technique besides not eliminating“veining” in some cases, has as a disadvantage that the iron oxidereduces the mechanical resistance of the core and moreover the formationof feyalite increases the tendency to penetration, causing the externalsurface of the piece obtained to present irregularities, which should betreated later.

Use of Wood Flours and Coal Powder

According to this technique, wood flour or coal powders are added inproportions varying from 1 to 3%. These flours burn during melting,hence leaving free gaps distributed throughout the volume of the core,permitting that the expansion of the silica is produced in these gapswithout the need to increase the external size, hence avoiding theappearance of fissures provoking “veining”. The main disadvantage ofthis technique is that when the flours burn, a large amount of gas isproduced which, on circulating, may result in dimensional problems inthe pieces obtained. Likewise, with this type of additive, a reductionin the mechanical resistance of the cores is produced.

Use of Titanium Oxide as an Additive

This new technique described in the U.S. Pat. No. 4,735,973, is based onthe use of titanium oxide additives, the additive being present atpercentages varying between 0.5 and 5% of the total amount of sand andsaid additive containing between 15 and 95% titanium oxide. With thistechnique, thermal expansion is reduced, preventing, as a result“veining”, maintaining the mechanical resistance of the cores and notproducing an increase in gas production. The disadvantage of thistechnique lies in the fact that the cores obtained possesses a certaintendency to penetration, it being necessary to apply paints or othertreatments on the surface of the cores obtained before proceeding tomelting the piece.

Use of Natural Sands of Low Expansion

This new technique uses for the formation of the core, special sands ofthe rounded of sub-angular silica type, chromate sands, zirconium sandsand olivine sands, which, due to their different degrees of thermalexpansion, result in the reduction of “veining”, and even to its totalelimination. The basic disadvantage of this technique is the high costof this type of sand, with the consequent increase in the cost to obtainthe cores.

Use of Electrofused Sands of Low Expansion

According to this technique, the silica sand normally used for themanufacture of cores is melted in electric ovens, until obtaining a kindof paste without expansion capacity. Then, the paste obtained is grounduntil obtaining a sand powder which is mixed approximately at 50% withsilica sand. In this way, the expansion of the core is avoided, sincethe powder obtained from the silica paste does not have a capacity forexpansion and hence, neither produces fissures nor the correspondingveining. The basic disadvantage of this technique is the greatercomplexity of the production process, which makes the final cost toobtain the cores more expensive.

As may be appreciated, the techniques normally used to prevent theformation of “veining” consist either in the use of additives (ironoxide, titanium oxide, wood flours and coal powder) or in the use ofspecial sands (natural sands of low expansion or electrofused sands oflow expansion).

Now it has been found that it is possible to improve the quality of theiron casting by using cores or molds manufactured with molding sandsincorporating hollow microspheres of aluminum silicate.

As a result, a purpose of this invention comprises a molding sand forcasting which incorporates hollow microspheres of aluminum silicate.

An additional purpose of this invention comprises a process tomanufacture cores or chill molds including the use of the molding sandindicated above. The resulting cores and molds also comprise a purposeof this invention.

Another additional purpose of this invention comprises a process tomanufacture iron casting including the use of the cores or moldsmentioned above. The resulting iron casting also comprises a purpose ofthis invention.

SUMMARY OF THE INVENTION

The invention provides a molding sand for casting which incorporateshollow microspheres of aluminum silicate in an amount between 1 and 30%by weight with respect to the total amount of molding sand.

The molding sand, purpose of this invention, is suitable to manufacturecores and chill molds which, in turn, may be used in the manufacture ofiron casting.

The use of hollow microspheres of aluminum silicate prevents theappearance of fissures during core expansion, but without increasing gasproduction and maintaining the mechanical properties of the coreobtained. During melting of the piece, the expansion of the silica inthe molding sand does not cause an increase of the core, but theexpansion is absorbed by the internal spaces of the hollow microspheres,by which the appearance of fissures on the core surface is totallyprevented and, as a result, “veining”.

With the molding sand of the invention, cores or molds are obtained oflesser density, by which gas production is reduced, but withoutdecreasing its mechanical resistance. Likewise, the penetration of thepiece obtained is reduced, due to the fact that the hollow microspheresof aluminum silicate cover the interstitial spaces of the core producingan effect similar to that of paint, improving the surface of the pieceobtained. Therefore, the quality of the resulting iron casting isimproved due to the reduction of the defects caused by core expansionand gas production.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a bar diagram in which the “veining” effect is seen fordifferent techniques of core shaping, position 04 corresponding to thetechnique based on the use of a molding sand of the invention containing10% by weight, of hollow microspheres of aluminum silicate.

FIG. 2 shows a bar diagram in which the mechanical resistance obtainedis seen according to the different techniques of core manufacture, theposition 04 corresponding to the technique based on the use of a moldingsand of the invention containing 10% by weight of hollow microspheres ofaluminum silicate.

FIG. 3 shows a bar diagram in which the density of the cores obtained isshown, according to the different manufacturing techniques.

FIG. 4 shows a comparative diagram of “veining” and penetration obtainedwith molding sands containing hollow microspheres of aluminum silicate(invention) and molding sands containing titanium oxide according theU.S. Pat. No. 4,735,973.

FIG. 5 shows a bar diagram in which the tensile strength of coresobtained with molding sands of this invention is shown, containingdifferent percentages of hollow microspheres of aluminum silicate, thecurves corresponding to the tensile strength at the exit of the box,after 24 hours and with a relative humidity of 100% being represented.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides a molding sand for casting incorporating hollowmicrospheres of aluminum silicate at an amount between 1 and 30% byweight with respect to the total amount of sand, preferably between 5and 25% and more preferably, between 10 and 20%, by weight.

Preliminary tests intended to prevent the formation of “veining” on theiron casting surface showed the possibility of using hollow microspheresof aluminum silicate as an additive for molding sands destined tomanufacture cores and chill molds.

Further tests permitted the verification that good results are obtainedwhen the hollow microspheres of aluminum silicate used have an aluminumcontent between 15 and 45% by weight, based on the weight of the hollowmicrospheres of aluminum silicate, preferably between 20 and 35% byweight.

For their use in this invention, all kinds of hollow microspheres ofaluminum silicate may be used, preferably those satisfying theaforementioned characteristics, such as those marketed by the PQCorporation under the trade mark Extendospheres, and those marketed byMicrofine Minerals Limited under the trade mark Metaspheres 50. In Table1, the main characteristics of the different microspheres used in thetests carried out are indicated.

Contrary to that expected, it was surprising to verify that the hollowmicrospheres of aluminum silicate of the best quality, understanding assuch those microspheres with a relatively high aluminum content,typically between 35 and 45% by weight, give worse results than whenhollow microspheres of aluminum silicate of less quality are used, thatis, with an aluminum content less than 35% by weight.

The tests performed with different hollow microspheres of aluminumsilicate, incorporated at different proportions to the molding sand haveshown that, surprisingly, the microspheres with a low content inaluminum (25-33%) give, in general, the best results regarding “veining”and penetration, in turn maintaining the mechanical properties of thecore obtained, moreover observing that an increase in the percentage ofaluminum in the microspheres does not imply an improvement in theresults of said effects (“veining” and penetration), but, on occasions,the opposite occurs [see Table 5, (Example 5)].

Moreover, the studies performed showed that the best results regardingveining and penetration do not only depend on the aluminum content, butother factors also have an influence, such as the size of themicrospheres and the thickness of their walls. Particularly, it has beenobserved that hollow microspheres of aluminum silicate are suitablehaving a wall thickness between 3 and 10% of the microsphere diameterand a particle size between 10 and 350 micrometers (μm).

As may be seen in Table 4 (example 4), the microspheres giving the bestresults are those identified as Metaspheres 50 and Extendospheres SG,since they have a crushing strength of 189.37 kg/cm² (2.700 psi) with analuminum content between 25 and 30% by weight, a wall thickness of 5%,with respect to the particle diameter (Extendospheres SG) and from 3 to7% with respect to the diameter of the particle (Metaspheres 50), and anaverage particle size of 150 μm (Extendospheres SG) and between 10 and250 μm (Metaspheres 50).

The molding sand of the invention may also contain other conventionalcomponents, like casting aggregates, binders and other optionalcomponents used in this sector of the technique.

The invention also provides a process to manufacture a core or chillmold by means of a cold process comprising:

(A) introducing the molding sand, purpose of this invention, into a moldto form a core or non-cured mold;

(B) placing said core or non-cured mold of stage (A) into contact with agaseous cured catalyst;

(C) permitting said core or non-cured mold resulting from stage B) tocure until said core or mold may be handled; and

(D) separating said core or mold from the mold.

In another embodiment, the invention also provides a process tomanufacture iron casting comprising:

(A) inserting the core or mold manufactured from the molding sand,purpose of this invention, in a casting device;

(B) pouring the metal, in a liquid state, in said casting device;

(C) letting the metal poured into the casting device cool and solidify;and

(D) separating the molten metal piece from the casting device.

The following examples serve to illustrate the invention. In Table 1,the main characteristics of the hollow microspheres of aluminum silicateused in the execution of these examples are shown.

TABLE 1 Characteristics of different hollow microspheres of aluminumsilicate Aluminum Particle Crushing Softening content Wall sizeresistance point Product (%) thickness (μm) (kg/cm²) (° C.)Extendospheres 43, 3 = 10% Ø 10-300 562, 48 1.800 SLG Extendospheres 43,3 = 10% Ø 10-180 562, 48 1.800 SL180 Extendospheres 43, 3 = 10% Ø 10-150562, 48 1.800 SL150 Extendospheres 25-30 = 10% 10-300 189, 37 1.200- SG(radio) (media 130) 1.350 Extendospheres 25-30 = 10% 10-350 189, 371.200- XEG (radio) (media 162) 1.350 Extendospheres 15 100 7, 03 1.000XOL200 (media) Metaspheres 50 26-33 3-7% Ø 10-250 196, 8- 1.200- 1.968,1 1.350 Extendospheres is a trade mark of The Pq Corporation Metaspheresis a trade mark of Microfine Minerals Ltd.

EXAMPLE 1 Study of the Use of Hollow Microspheres of Aluminum Silicateas an Additive For Molding Sands

To assess the possible use of hollow microspheres of aluminum silicateas an additive for molding sands, destined to manufacture casting cores,on the one hand some cores were formed using different resins andconventional additives, and on the other hand, other cores from amolding sand, to which hollow microspheres of aluminum silicate had beenadded, then studying “veining” and the tensile strength of the coresobtained. The techniques used to manufacture the different cores wereconventional for each case.

The distinctive components for the different mixtures used tomanufacture the cores, are summarized below (Table 2). In all the cases,2% resin was used. The catalyst used in preparation 02 and 03 was SO₂(gas) whilst in the remaining preparations, the catalyst used wasgaseous methylethylamine (DMEA).

TABLE 2 Starting mixtures Preparation Resin Molding sand 01 PhenolicSilica sand (*) urethane 02 Epoxy acrylic Silica sand (*) 03 AcrylicSilica sand (*) 04 Phenolic Silica sand (*) + 10% hollow urethanemicrospheres of aluminum silicate (invention) 05 Phenolic Recoveredfuranic sand urethane 06 Phenolic 70/30 silica sand urethane(*)/Chromite 07 Phenolic 50/50 silica sand urethane (*)/Chromite 08Phenolic Silica sand (*) + 2% BR-022 urethane 09 Phenolic Silica sand(*) + 2% coal urethane 10 Phenolic seggar clay urethane 11 Phenolic50/50 electrofused silica urethane 12 Phenolic treated olivine urethane13 Phenolic Thermally recovered sand urethane 14 Phenolic Silica sand(*) + 10% Veinseal urethane 14000 (*): Silica sand AFA = 50 roundedtype, % Si >97%

Once the piece was prepared, the results were studied the value “10” tothe maximum value of “veining” and a value “0” to the minimum value of“veining”. Besides “veining”, tensile strength was evaluated.

In FIGS. 1 and 2, bar diagrams are shown indicating the “veining” effectand tensile strength of the cores obtained. In the position 04, theproperties obtained with the core obtained from molding sand containingmicrospheres of aluminum silicate at a percentage of 10% are shown, itbeing possible to observe the total absence of the “veining” effect andsome good tensile strength properties.

EXAMPLE 2 Density of Different Cores

The density of different cores obtained according to differentmanufacturing techniques has been determined including, for comparativepurposes, a core manufactured from a molding sand containing hollowmicrospheres of aluminum silicate, purpose of this invention. The cores,whose density has been evaluated were prepared using the sands andadditives listed below:

[1]: Additives of titanium oxide [U.S. Pat. No. 4,735,973] (Veinseal).

[2]: Hollow microspheres of aluminum silicate (Invention).

[3]: Rounded silica.

[4]: Sub-angular silica.

[5]: 70/30 Rounded silica/chromite.

[6]: 90/10 Silica/Additive of titanium oxide [U.S. Pat. No. 4,735,973](Veinseal).

[7]: 90/10 Silica/Hollow microspheres of aluminum silicate (invention).

The results obtained are shown in FIG. 3, where it may be appreciatedthat the cores manufactured from molding sands containing hollowmicrospheres of aluminum silicate, have a very reduced density withrespect to that of the other cores, a density permitting the reductionof gas production and penetration in the piece obtained.

EXAMPLE 3 Comparative Example

Some cores were prepared as from some molding sands containing differentamounts (0, 5%, 10% y 20%) of an additive selected between:

(i) hollow microspheres of aluminum silicate, and

(ii) additives of titanium oxide according to the North American PatentU.S. Pat. No. 4,735,973 (Veinseal), and the effect of the same, both on“veining” and penetration has been evaluated.

The cores were prepared by mixing the sand (C-55) with 0.5%, 10% or 20%by weight of the additive in question and to the resulting mixtures, thesuitable resins were added, formed and cured.

Once the different pieces were prepared, the results were evaluated,giving the value “10” to the maximum level of “veining” and penetrationand the value “0” to the minimum level of veining and penetration. Todetermine the penetration of the metal in the mold, the test“Penetration 2×2 test casting” [AFS Transactions] was used, in which thecavities of the core in the test mold were visually examined for theexistence of metal penetration.

The results obtained are shown in FIG. 4, where it is clearly seen thatthe “veining” in both techniques is very similar and is graduallyreduced until it disappears when the percentage of additive graduallyincreases until reaching 10%. However, the penetration using additivesof titanium oxide increases as the percentage of additive increases,whilst when using hollow microspheres of aluminum silicate as anadditive, the penetration remains constant and at a very reduced level.

EXAMPLE 4 Preparation of Cores Using Hollow Microspheres of AluminumSilicate as an Additive

Some cores were prepared (crushing trials) consisting of molding sand,to which different amounts (0.5%, 10% and 20%) of hollow microspheres ofaluminum silicate had been added, and the incidence thereof on thetensile strength of the cores obtained was evaluated.

The test pieces were prepared by mixing the sand (C-55) with 0.5%, 10%or 20% by weight of some hollow microspheres of aluminum silicate and tothe resulting mixture, the appropriate resin mixture was added With themixture obtained, the crushing trials were prepared which were curedwith the suitable gas.

The results obtained are collected in FIG. 5, where the tensile strengthof the cores obtained with different percentages of the additive,purpose of the invention, are shown, representing the curvescorresponding to the tensile strength at the exit of the box, after 24hours and with a relative humidity of 100%.

By means of a process similar to the above, some cores were prepared asfrom the molding sands indicated in Table 3, obtained by mixing the sand(C-55) with 0.5%, 10% or 20% by weight of hollow microspheres ofaluminum silicate. In all cases, 1% Isocure® 325 (Ashland) resin and 1%Isocure® 625 (Ashland) resin, and DMEA as a catalyst were used.

TABLE 3 Molding sands C-55 sand Additive Composition (% by weight) (% byweight) I 100   0 II 95  5 III 90 10 IV 80 20

The cores obtained were submitted to some abrasion resistance tests(Scratch Hardness, SH) and tensile strength tests (Tensile Hardness,TS). The results obtained are shown in Table 4.

TABLE 4 Mechanical resistances Resistance I II III IV composition TS SHTS SH TS SH TS SH 2 cc. 302 68 94 56 93 54 92 44 90  1 hour  76 95 72 9474 96 60 92 24 hours 88 98 95 97 98 97 85 96 1 h. Air 23 73 35 86 30 7926 74 and 24 h. 100% humidity Test piece 448.9 425.0 385.8 318.8 weight

The following examples were made with the purpose of selecting the mostsuitable hollow microspheres of aluminum silicate for their use as anadditive in molding sands.

EXAMPLE 5 Evaluation of Different Hollow Microspheres of AluminumSilicate as an “Anti-Veining” Additive

To evaluate the “anti-veining” behavior of different types ofmicrospheres of aluminum silicate, some test pieces for crushing testswere prepared, consisting of molding sand to which different amounts ofthe microspheres to be evaluated had been added.

The test pieces were prepared by mixing the sand (C-55) with 10% or 20%by weight of the microspheres and to the resulting mixture 0.75%Isocure® 325 (Ashland) and 0.75% Isocure® 625 (Ashland) were added. Withthe mixture obtained, some test pieces for crushing were made, gassingthem with Isocure® 720 (Ashland) Afterwards, they were placed in a moldfor their melting with gray iron at 1,420° C.

Once the piece had been cooled, the results were evaluated, giving thevalue “10” to the maximum level of “veining” and penetration and thevalue “0” to the minimum level of “veining” and penetration. Todetermine the penetration of the metal in the mold, the test“Penetration 2×2 test casting” [AFS Transactions] was used, in which thecavities of the core were examined in the test mold to visually examinedthe existence of metal penetration.

The results obtained are shown in Table 5, where it may be appreciatedthat the best results regarding “veining” and penetration (that is,those in which “veining” and penetration was obtained with a value ofzero or very near to zero) were obtained when using 20% by weight of thehollow microspheres of aluminum silicate with an aluminum contentbetween 25 and 33% (Extendospheres SG and Metaspheres SLG, SL180 andSL150, with an aluminum content near to 45% by weight) which gave theworse results in general.

TABLE 5 Study of “anti-veining” products Test pieces for crushingIsocure ® 325/ Isocure ® 625 (1.5% resin total) Test piece Test C-55weight SL SL Meta. XOL piece No. sand (g) 180 150 SLG 50 XEG SG 200Veining Penetration Control A 100 175, 8 — 8 2  1  90 151, 5 10 — — — —— — 9 2  2  80 122, 2 20 — — — — — — 9 2  3  90 150, 1 — 10 — — — — — 92  4  80 124, 3 — 20 — — — — — 9 4  5  90 147, 2 — — 10 — — — — 9 1  6 80 121, 0 — — 20 — — — — 10  0  7  90 150, 0 — — — 10 — — — 4 3  8  80123, 2 — — — 20 — — — 0 0  9  90 144, 6 — — — — 10 — — 2 2 10  80 117, 0— — — — 20 — — 0 1 11  90 147, 0 — — — — — 10 — 2 0 12  80 122, 0 — — —— — 20 — 0 0 13  90 175, 4 — — — — — — 10 9 2 14  95 176, 0 — — — — — — 5 10  5 [SL 180, SL150, SLG, XEG, SG and XOL200 are different types ofExtendospheres; Meta. 50: Metaspheres 5° ; (Table 1)]

EXAMPLE 6 Evaluation of the Mechanical Resistance of “Anti-Veining”Additives

To evaluate the mechanical resistance of different types of microspheresof aluminum silicate, some tensile strength test pieces were prepared,consisting of sand to which different amounts of the microspheres to heevaluated had been added.

The test pieces were prepared by mixing the sand (C-55) with 10% or 20%by weight of the microspheres and to the resulting mixture, 0.75%Isocure® 325 (Ashland) and 0.75% Isocure® 625 (Ashland) were added. Thecatalyst used was DMEA. With the mixture obtained, some tensile strengthtest pieces were made, which were subjected to abrasion resistance (SH)and tensile strength (TH) tests. The result obtained are shown in Table6, where it is observed that in spite of the good results obtained inthe “veining” and penetration effects, also satisfactory mechanicalresistances were obtained, for the cores prepared from the molding sandsof the invention.

TABLE 6 Study of the mechanical resistances of agglomerated products(sand/microspheres) ISOCURE ® 325 ISOCURE ® 325 ISOCURE ® 325 ISOCURE ®325 ISOCURE ® 325 ISOCURE ® 325 Resin ISOCURE ® 625 ISOCURE ® 625ISOCURE ® 625 ISOCURE ® 625 ISOCURE ® 625 ISOCURE ® 625 Amount 1.5 1.51.5 1.5 1.5 1.5 Catalyst DMEA DMEA DMEA DMEA DMEA DMEA Product 100% C-5590% C-55 80% C-55 90% C-55 80% C-55 80% C-55 Agglomerate (Control) 10%EX XEG 20% EX XEG 10% EX SG 20% EX SG 20% MS 50 TS SH TS SH TS SH TS SHTS SH TS SH 3 cc. 3′ 50 92 67 92 56 90 63 92 57 91 50 90 1 hour 73 96 7294 58 91 70 93 59 93 48 73 24 hours 83 97 78 94 63 92 86 95 73 95 66 871 h air & 24 h 60 94 61 89 59 90 70 93 60 90 49 83 100% humidity Density228.3 186.0 153.3 192.3 156.0 156.0 3 test pieces 6 hours of life inbank TS SH TS SH TS SH TS SH TS SH TS SH 3 cc. 3′ 38 83 40 81 25 43 3880 22 49 15 32 1 hour 46 91 44 83 26 44 40 82 25 49 12 31 24 hours 55 9448 86 30 47 48 85 29 50 13 40 1 h air & 24 h 48 92 38 81 23 40 44 81 2040 11 32 100% humidity [MS: Metaspheres; EX: Extendospheres; TS: tensilestrength; SH: abrasion resistance]

EXAMPLE 7 Evaluation of Mechanical Resistances of Different HollowMicrospheres of Aluminum Silicate

To evaluate the mechanical resistance of different hollow microspheresof aluminum silicate at 100%, some tensile strength test pieces wereprepared, by mixing the microspheres (100%) to be evaluated with 3%Isocure® 323 (Ashland) and 3% Isocure® 623 (Ashland) With the mixturesobtained, some tensile strength test pieces were made which were gassedwith Isocure® 702 (Ashland) The test pieces obtained were submitted toabrasion resistance (SH) and tensile strength (TH) tests. The resultobtained are shown in Table 7, where it may be appreciated that the bestresults were obtained with Extendospheres XEG microspheres, having anaverage particle size (162 μm) greater than the Extendospheres SGmicrospheres(130 μm).

TABLE 7 Study of mechanical resistances of different additives (withIsocure) used in the manufacture of sleeves METASPHERES EX SL180 EXSL150 EX SLG 50 EX SG EX XEG Microsphere ISOCURE ISOCURE ISOCURE ISOCUREISOCURE ISOCURE Resin 323/623 323/623 323/623 323/623 323/623 323/623Resin amount 6 6 6 6 6 6 Catalyst DMEA DMEA DMEA DMEA DMEA DMEA TS  SHTS  SH TS  SH TS  SH TS  SH TS  SH 4 cc. 3′ 49  80 49  82 47  78 48  8136  71 60  83 1 hour 63  86 52  84 66  87 43  80 50  80 66  83 24 hours70  92 67  86 67  90 40  79 72  94 78  95 1 h air & 24 h 60  85 43  7763  87 38  75 54  79 63  94 100% humidity [Ex: Extendospheres; TS:tensile strength; SH: abrasion resistance]

What is claimed is:
 1. A composition for the manufacture of cores andchill molds which comprises a molding sand, a resin and hollowmicrospheres of aluminum silicate, said hollow microspheres of aluminumsilicate being present in the composition in an amount between 1 and 30%by weight of the total amount of the composition and having an aluminumcontent between 15 and 45% by weight of the microspheres to absorbexpansion when the core or chill mold is heated by a molten castingmetal and thereby to prevent formation of fissures in the core or chillmold and veining of a cast article.
 2. A composition according to claim1, wherein said hollow microspheres of aluminum silicate are present inan amount of 5 and 25% by weight of the total amount of the composition.3. A composition according to claim 1, wherein said hollow microspheresof aluminum silicate have an aluminum content between 20 and 35% byweight, of the microspheres.
 4. A composition according to claim 1,wherein said hollow microspheres of aluminum silicate have a wallthickness between 3 and 10% of the microsphere diameter.
 5. Acomposition according to claim 1, wherein said hollow microspheres ofaluminum silicate have a particle size between 10 and 350 μm.
 6. A coldprocess for the manufacture of a core or chill mold comprising:introducing a composition for the manufacture of cores and chill moldsaccording to claim 1, in a forming mold to form a non-cured core ormold; contacting said non-cured core or mold with a gaseous curingcatalyst until said core or mold may be handled; and separating saidcore or chill mold from the forming mold.
 7. A core or chill moldproduced according to the process of claim
 6. 8. A process for themanufacture of an iron casting, which comprises: inserting a core orchill mold, according to claim 7, in a casting device; pouring an ironcasting composition in a liquid state, in said casting device; allowingthe casting composition poured into the casting device to cool andsolidify; and removing the casting composition thus solidified from thecasting device as the target iron casting.
 9. A method of producing aniron casting comprising: introducing a molding sand into a forming mold,said molding sand having a composition comprising: sand, a resin andhollow microspheres of aluminum silicate, said hollow microspheres ofaluminum silicate being present in the composition in an amount between1 and 30% by weight of the total amount of the composition and having analuminum content between 15 and 45% by weight of the microspheres,forming a non-cured core or mold in said composition, contacting saidnon-cured core or mold with a gaseous curing catalyst until said core ormold may be handled, inserting the core or mold in a casting device,pouring an iron casting composition in a liquid state, in said castingdevice; allowing the iron casting composition poured into the castingdevice to cool and solidify; and removing the thus solidified ironcasting from the casting device, said iron casting being free of surfacedefects including veining due to smoothness of the core or mold as aresult of absorption of expansion by said microspheres when the core ormold is heated by the casting metal, whereby the iron casting is thetarget product as is without need for machining.
 10. A method ofreducing surface defects including veining in the surface of an ironcasting produced by a method comprising: introducing a moldingcomposition into a forming mold, forming a non-cured core or mold ofsaid composition, contacting said non-cured core or mold with a gaseouscuring catalyst until said core or mold may be handled, inserting thecore or mold in a casting device, pouring an iron casting composition ina liquid state, into said casting device; allowing the iron castingcomposition poured into the casting device to cool and solidify; andremoving the thus solidified iron casting from the casting device, saidiron casting being free of surface defects including veining by formingsaid molding composition as follows: sand, a resin and hollowmicrospheres of aluminum silicate, said hollow microspheres of aluminumsilicate being present in the molding composition in an amount between 1and 30% by weight of the total amount of the molding composition andhaving an aluminum content between 15 and 45% by weight of themicrospheres the presence of said microspheres in the moldingcomposition as aforesaid producing a smoothness of the core or mold as aresult of absorption of expansion by said microspheres when the core ormold is heated by the casting metal, whereby the iron casting isobtained as the target product, as is, without need for machining.